Most of the time I’ve mapped a feature without adding a source, it’s because the geometry comes from one of the built-in imagery layers. The changeset’s imagery_used tag records the layer(s) automatically. In the past, some OSM editors like Potlatch made it easy to tag the background layer as a source on the feature, but these tags usually got stale very fast and rarely carried any real meaning. Not only can other mappers tweak the geometry without touching the source, but the contents of the background layer itself are also ephemeral, subject to change at any time without a backup. I think this risk distinguishes these sources from the static sources used to tag attributes or map older geometries.
Even when I trace from background imagery, I make a best effort to tag the sources I used for attributes such as start_date and name. But sometimes I omit the dates because I’ve only been able to ascertain them by inference. For example, I’ve determined the start date of this on-ramp via the dates of the road alignment and overpass it connects. I probably should add a note to that effect, but I wouldn’t want to matter-of-factly source the date to a source that remains silent on the on-ramp. Putting words in someone’s mouth is almost as bad as plagiarizing them.
By the way, there’s a proposal to warn when citing certain sources too vaguely: